As invisible information printed sheets for lotteries, etc., conventionally, there are generally used sheets obtained by printing win- or losing-showing information of letters, numbers, patterns, etc., on sheets such as sheets of paper and further coating concealing layers thereon to bring the information into an invisible state. Specifically, there can be referred to a sheet that is obtained by printing visible information, etc., on a sheet of paper, etc., further printing information that is to be invisualized, then, forming a peel agent layer so as to cover the information that is to be invisualized and forming a masking concealing ink layer in a sliver color, etc., thereon, and it ensures that when the peel agent layer is scratched off with a coin, etc., the invisible information appears. Available as the above invisible information printed sheet are a sheet obtained by forming a concealing layer such as a concealing ink layer, a metal deposition layer, etc., on a surface having visible information printed thereon and a sheet obtained by forming the concealing layer on the opposite surface. In the latter invisible information printed sheet, it is a precondition that the support thereof should be transparent (JP8-150796A).
Further, there is proposed a printed matter obtained by printing information to be invisualized on a support with an electron-accepting color developer and forming a scratch layer so as to cover the same, the scratch layer having capsules containing an electron-donating colorless or light-color dye precursor dispersed therein each (JP10-16386A).
Since, however, the invisible information printed sheet using a concealing layer such as a concealing ink layer has a defect that scratching dust that occurs during the removal of the concealing layer is a mote, the field of its use is limited. Due to the necessity of increasing the thickness of the concealing layer for concealing information to be invisualized, it has a defect that the concealing layer is liable to peel off due to its frictional contact or its contact to a sharply pointed article. Further, the concealing ink has a dark color or the color of a metallic luster, and it is hence liable to feel dark or liable to have a peculiar appearance, which is liable to pose a problem with regard to a design.
On the other hand, the invisible information printed sheet using capsules containing an electron-donating colorless or light-color dye precursor each and an electron-accepting color developer has a problem that soiling may easily occur or that the invisible information may be easily visualized, since capsules containing the electron-donating colorless or light-color dye precursor may be broken in a processing step or due to a frictional contact during handling. Further, it also has a problem that a portion on which pressure like a writing pressure is exerted develops a color even without any frictional contact. Further, since it uses the copying paper technology that takes no scratching into consideration, an information printed portion may be also scratched off when it is scratched carelessly with a strong force. In the use of an invisible information printed sheet, it is liable to be used roughly in an attempt to know the invisible information quickly, so that it cannot be said to be suitable as an invisible information printed sheet.
Further, there is another invisible information printed sheet that is obtained by printing letters or a pattern on a support by the use of an ink containing, as a main component, a pigment (silicon dioxide, titanium oxide, aluminum oxide, etc.) having higher hardness than a metal material constituting a coin, and that makes the information visible when the metal material is scraped off by rubbing a coin thereon (JP6-78039B).
However, it inconveniently requires a tool such as a coin, etc., and in view of safety and healthfulness, it is not desirable to allow children of tender years to use coins, since they may swallow them by accident or make hands dirty. In many uses, users are grownups, and there can be supposed situations in which children and aged persons are around to play with it or touch it, but no consideration is given to such cases. In the above method, further, the developed color density of printed information during visualization is low, and there can be obtained only a developed color density that is barely visually recognizable.
The above defects, in particular, the problem of scratching dust can be overcome by an ink containing a varnish and containing a colorless or light-color dye precursor and an electron-accepting developer in the form of solid particles each in the varnish, and an invisible information printed sheet that is printed with the ink. However, the existing situation is that invisible information printed sheets are facing a high degree of demands one after another (JP 2006-199887A).